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Voltage Limiting Devices (VLDs) in DC Railways

Electric railways are using rails as a robust electrical conductor for return currents from the trainsets back to the power supply substations.

As the electrical conductivity of even a massive rail is limited (for example, the rails feature a specific value of electrical resistance and inductance), and the rails cannot be perfectly isolated from the earth (in addition to regular leakage currents arising in the track via limited impedance of track isolation a significant occurrence of current leakage can be observed, for example, at places with contaminated ballast, railroad crossings), a part of the return current, also known as stray current, escapes from the rails to the environment and flows outside of the rails through parallel paths with the lowest resistance, for example through the surrounding soil and, in particular, nearby conductive structures back to traction power substations, in accordance with the principles of Ohm's and Kirchhoff's circuit laws.

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